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ScienceWeek
MATERIALS SCIENCE: ON NUCLEIC ACID NANOTECHNOLOGY
The following points are made by Hao Yan (Science 2004 306:2048):
1) Nucleic acids are best known as the carriers of genetic information, but they are also a versatile material for designing nanometer-scale structures, because nucleic acid sequences can be designed such that the strands fold into well-defined secondary arrangements. In 1982, Seeman [1] first proposed using branched DNA building blocks to construct ordered arrays. In recent years, DNA has been shown to be an ideal molecule for building micrometer-scale arrays [2,3] with nanometer-scale features. DNA can also be used to make nanometer-scale materials with moving parts, such as nanotweezers [4].
2) Currently, two major challenges face nucleic acid-based nanotechnology: (a) to produce complex superstructures from simple molecular building blocks, and (b) to perform controlled mechanical movements in molecular devices. Liao and Seeman [5] have presented a DNA device that can program the synthesis of linear polymers through positional alignment of reactants. Chworos et al [6] have used rationally designed RNA building blocks as jigsaw puzzle pieces that direct pattern formation. The two studies demonstrate that it will be feasible to build functional materials and devices from "designer" nucleic acids.
3) Nanotechnology researchers have sought to mimic nature's biological motors to create nanometer-scale machines that can function in an engineered environment. Liao and Seeman [5] take an important step in this direction with a device that mimics the translational capabilities of the ribosome. The device consists of two subsections, each with two structural states. Different pairs of DNA "set strands" can be added or removed to bring the device into any one of four states. Each state allows the positional alignment of a specific pair of DNA motifs that are selected from a pool. The pairs bear polymer components that can then be fused in a specific order.
4) As proof of principle, Liao and Seeman [5] used DNA as the polymer that is aligned, and enzymatic ligation to fuse the polymers. Positional synthesis with the prototype device thus results in four different DNA strands, each containing a defined sequence. In this ribosome-like DNA device, there is no complementary relationship between the signal sequence and the products. Furthermore, all polymer reactants exist simultaneously in one solution. These features make the device appealing for building nanometer-scale machines that control massively parallel chemical synthesis. Kanan et al (2004) have shown that DNA-templated organic synthesis can be used to discover new bond-forming chemical reactions.
References (abridged):
1. N. C. Seeman, J. Theor. Biol. 99, 237 (1982)
2. E. Winfree, F. Liu, L. A. Wenzler, N. C. Seeman, Nature 394, 539 (1998)
3. H. Yan, S. H. Park, G. Finkelstein, J. H. Reif, T. H. LaBean, Science 301, 1882 (2003)
4. B. Yurke, A. J. Turberfield, A. P. Mills Jr., F. C. Simmel, J. E. Neumann, Nature 406, 605 (2000)
5. S. Liao, N. C. Seeman, Science 306, 2072 (2004)
Science http://www.sciencemag.org
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Related Material:
ON SUPRAMOLECULAR ASSEMBLY ON SURFACES
The following points are made by G.M. Credo et al (J. Am. Chem. Soc. 2002 124:9036):
1) Metal-molecule-metal junctions have been used to elucidate single-molecule properties in organic monolayers that are applicable in molecular electronics. For example, pore-based sandwich structures(1,2), mechanical break junctions(3), and Hg drop electrode top contacts(4,5) have been used to characterize the current-voltage properties of organic monolayers. Studies have also employed scanning tunneling microscopy (STM) and conducting atomic force microscopy (cAFM) to probe the current-voltage properties of molecular junctions with more limited contact areas. In particular, reports of STM studies on redox-active molecular monolayers have described the use of electroactive moieties in molecular junctions to facilitate nonlinear current-voltage behavior. In a recent example, the nonlinear current-voltage phenomenon of negative differential resistance (NDR) was observed in an electroactive, ferrocene-terminated self-assembled monolayer (SAM). The identification of nonlinear current-voltage properties such as NDR for individual molecules expands the potential applicability of molecule-scale components from use as conductive wires to multistate molecular switches.
2) Chemical self-assembly is an attractive method for reversibly constructing well-defined supramolecular systems with properties defined by their molecular components. In particular, hydrogen bonding is a familiar construction motif in natural systems and has been used to assemble functional nanostructures, such as metal nanoparticle-based networks. Applying these concepts, noncovalent self-assembly provides a potential method to install and subsequently remove electroactive functionality in molecular electronics systems. To explore this possibility, the authors report they patterned a footprint region for molecular assembly on a surface featuring a recognition-element-terminated thiol. The authors then used moieties featuring complementary recognition to tune the current-voltage properties of the patterned region. In the current work, the authors used an STM tip to pattern and probe molecular assemblies and independently verified the hydrogen bond-mediated assembly process using bulk electrochemical and spectroscopic techniques.
3) In summary: The authors report that molecules capable of complementary hydrogen bonding were used to control the noncovalent self-assembly and electronic properties of a chemically well-defined surface mesostructure. In this work, the authors patterned a footprint region for molecular assembly on a surface and used moieties featuring complementary recognition to tune the current-voltage properties of the patterned region. With the appropriate functionalities on the complementary moieties, the authors were able to increase and decrease the observed conductance in surface-bound mesoscale structures imaged by scanning tunneling microscopy (STM).
References (abridged):
1. Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2000, 77, 1224-1226.
2. Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550-1552.
3. Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252-254.
4. (a) Holmlin, E. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Rampi, M. A.; Terfort, A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075-5085. (b) Holmlin, R. E.; Ismagilov, R. F.; Haag, R.; Mujica, V.; Ratner, M. A.; Rampi, M. A.; Whitesides, G. M. Angew. Chem., Intl. Ed. 2001, 2316-2320.
5. Selzer, Y.; Salomon, A.; Ghabboun, J.; Cahen, D. Angew. Chem., Int. Ed. 2002, 41, 827-830.
J. Am. Chem. Soc. http://pubs.acs.org/JACS
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Related Material:
SUPRAMOLECULAR CHEMISTRY
The following points are made by S.T. Nguyen et al (Proc. Nat. Acad. Sci. 2001 98:11849):
1) For over 100 years, chemistry has focused primarily on understanding the behavior of molecules and their construction from constituent atoms, and our current level of understanding of molecules and chemical construction techniques has given us the confidence to tackle the construction of virtually any molecule, be it biological or designed, organic or inorganic, monomeric or macromolecular in origin.
2) During the last few decades, chemists have extended their investigations beyond atomic and molecular chemistry into the realm of "supramolecular chemistry". Terms such as "molecular self-assembly", "hierarchical order", and "nanoscience" are often associated with this area of research.
3) In general, supramolecular chemistry is the study of interactions between, rather than within, molecules -- in other words, chemistry using molecules rather than atoms as building blocks. Whereas traditional chemistry deals with the construction of individual molecules (1 to 100 angstroms length scale) from atoms, supramolecular chemistry deals with the construction of organized molecular "arrays" with much larger length scales (1 to 100 nanometers).
4) In classical molecular chemistry, strong association forces such as covalent and ionic bonds are used to assemble atoms into discrete molecules and hold them together. In contrast, the forces used to organize and hold together supramolecular assemblies are weaker non-covalent interactions, such as hydrogen bonding, polar attractions, van der Waals forces, and hydrophilic-hydrophobic interactions.
Proc. Nat. Acad. Sci. http://www.pnas.org
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